[Warning] 150-webalizer.inc.php - exec() expects at most 3 parameters, 4 given

Hello,
When I run the following command, I get warnings. How can I fix it?

Code:

root@hosting:/usr/local/ispconfig/server# php cron_debug.php --cronjob=150-webalizer.inc.php
PHP Warning:  exec() expects at most 3 parameters, 4 given in /usr/local/ispconfig/server/lib/classes/cron.d/150-webalizer.inc.php on line 131
PHP Warning:  exec() expects at most 3 parameters, 4 given in /usr/local/ispconfig/server/lib/classes/cron.d/150-webalizer.inc.php on line 131
PHP Warning:  exec() expects at most 3 parameters, 4 given in /usr/local/ispconfig/server/lib/classes/cron.d/150-webalizer.inc.php on line 131
PHP Warning:  exec() expects at most 3 parameters, 4 given in /usr/local/ispconfig/server/lib/classes/cron.d/150-webalizer.inc.php on line 131
finished.

This is the 131. line:

PHP:

exec('chown -R ?:? ?', $username, $groupname, $statsdir);